Experiments#

Enable and try out some experimental features.

• Type: object

experiments.asyncWebAssembly#

• Type: boolean
• Default: false

Support the new WebAssembly according to the updated specification, it makes a WebAssembly module an async module. And it is enabled by default when experiments.futureDefaults is set to true.

experiments.outputModule#

• Type: boolean
• Default: true

Once enabled, Rspack will output ECMAScript module syntax whenever possible. For instance, import() to load chunks, ESM exports to expose chunk data, among others.

module.exports = {
  experiments: {
    outputModule: true,
  },
};

experiments.css#

• Type: boolean
• Default: Defaults to false for Rspack 1.x, true for Rspack 0.x

Once enabled, Rspack will enable native CSS support, and CSS related parser and generator options.

• module.parser["css/auto"]
• module.parser.css
• module.parser["css/module"]
• module.generator["css/auto"]
• module.generator.css
• module.generator["css/module"]

Basic example:

module.exports = {
  experiments: {
    css: true,
  },
};

experiments.futureDefaults#

Use defaults of the next major Rspack and show warnings in any problematic places.

module.exports = {
  experiments: {
    futureDefaults: true,
  },
};

experiments.topLevelAwait#

Enable support for top level await, top level await can only be used in modules with ModuleType is javascript/esm.

Enabled by default and can be turned off with this configuration.

experiments.lazyCompilation#

• Type: boolean, Object
• Default: false

type LazyCompilationOptions =
  | boolean
  | {
      backend?: {
        /**
         * A custom client.
         */
        client?: string;
        /**
         * Specify where to listen to from the server.
         */
        listen?: number | ListenOptions;
        /**
         * Specify the protocol the client should use to connect to the server.
         */
        protocol?: 'http' | 'https';
      };
      /**
       * Enable lazy compilation for entries.
       */
      entries?: boolean;
      /**
       * Enable lazy compilation for dynamic imports.
       */
      imports?: boolean;
      /**
       * Specify which imported modules should be lazily compiled.
       */
      test?: RegExp | ((m: Module) => boolean);
    };

Enable lazy compilation, which can greatly improve the dev startup performance of multi-page applications (MPA) or large single-page applications (SPA). For example, if you have twenty entry points, only the accessed entry points will be built. Or if there are many import() statements in the project, each module pointed to by import() will only be built when it is actually accessed.

If set to true, lazy compilation will be applied by default to both entry modules and modules pointed to by import(). You can decide whether it applies only to entries or only to import() through a configuration object. The entries option determines whether it applies to entries, while the import() option determines whether it applies to import().

const isDev = process.env.NODE_ENV === 'development';

module.exports = {
  experiments: {
    // only enabled in dev mode
    lazyCompilation: isDev,
  },
};

In addition, you can also configure a test parameter for more fine-grained control over which modules are lazily compiled. The test parameter can be a regular expression that matches only those modules that should be lazily compiled. It can also be a function where the input is of type 'Module' and returns a boolean value indicating whether it meets the criteria for lazy compilation logic.

experiments.lazyCompilation.backend.listen#

• Type: number, ListenOptions

type ListenOptions = {
  port?: number | undefined;
  host?: string | undefined;
  backlog?: number | undefined;
  path?: string | undefined;
  exclusive?: boolean | undefined;
  readableAll?: boolean | undefined;
  writableAll?: boolean | undefined;
  /**
   * @default false
   */
  ipv6Only?: boolean | undefined;
};

Exclude HMR client#

If you do not use Rspack's own dev server and instead use your own server as the dev server, you generally need to add another client modules in the entry configuration to enable capabilities such as HMR. It is best to exclude these client module from lazy compilation by configuring test.

If not excluded and lazy compilation of entry is enabled, this client will not be compiled when accessing the page for the first time, so an additional refresh is needed to make it take effect.

For example:

const rspack = require('@rspack/core');

const options = {
  experiments: {
    lazyCompilation: {
      test(module) {
        const isMyClient = module.nameForCondition().endsWith('dev-client.js');
        // make sure that dev-client.js won't be lazy compiled
        return !isMyClient;
      },
    },
  },
};
const compiler = rspack(options);

new compiler.webpack.EntryPlugin(compiler.context, 'dev-client.js', {
  // name: undefined means this is global entry
  name: undefined,
}).apply(compiler);

experiments.layers#

• Type: boolean
• Default: false

Controls whether to enable the layer feature. Layers can add an identifier prefix to all modules in a subgraph starting from a module in the module graph, to distinguish them from modules in different layers. For example:

The layer of the index.js module is by default null, and its identifier is ./index.js. If we set layer = 'client' for it, its identifier will become (client)/./index.js. At this point, the index.js modules in these two different layers will be treated as distinct modules, because their unique identifiers are different. As a result, the final output will include the artifacts of both modules.

By default, a module's layer is null, and it will inherit its parent module's layer. You can add a layer to an entry module using entryOptions.layer, and you can add a layer to matched modules using module.rule[].layer. Additionally, you can match based on the parent module's layer using module.rule[].issuerLayer.

experiments.rspackFuture#

• Type: object

• Default: See options down below for details

Used to control whether to enable Rspack future default options, check out the details here.

experiments.rspackFuture.bundlerInfo#

• Type:

  type BundlerInfo = {
    version?: string,
    bundler?: string,
    force?: ('version' | 'uniqueId')[] ｜ boolean;
  };

Used to inject the currently used Rspack information into the generated asset:

• version: Used to specify the Rspack version, defaults to the version field in @rspack/core/package.json.
• bundler: Used to specify the name of the packaging tool, defaults to rspack.
• force: Whether to force the injection of Rspack information, which will be added to chunk as a runtime module, and defaults to true to force injection. An array can be used to select the items to be forced injected。

The default injection can be disabled by setting force to false. Then injection will only occur when __rspack_version__ and __rspack_unique_id__ are detected in the code:

• __rspack_version__: Inject version information.
• __rspack_unique_id__: Inject the unique ID of the bundler.